Efficiency is not what is under discussion. What is under discussion is the actual lifetime cost of Solar as compared to the actual return over that lifetime.

No one in this discussion has actually mentioned coal (or any other fossil fuel) as I read it.
Nuclear and Wind power have been mentioned.
Wind is limited by and to appropriate locations, but as to cost and efficiency, it is a the top of the list.
Nuclear is limited by political will, and by poorly executed installation parameters. Otherwise, it really would be too cheap to meter. Think "test sites in Nevada" when it comes to waste, and think "Fast breeder reactor" when it comes to new fuel. All established technologies that if installed in an appropriate scale would be entirely safe.

Peter Wieck
Melrose Park, PA